Combined casing expansion/casing while drilling method and apparatus

ABSTRACT

A drilling assembly ( 2 ) comprises upper ( 4 ) and lower ( 6 ) drill strings with a fluid passage ( 80 ) therethrough for distributing fluid to a bottom hole assembly ( 10 ). The upper drill string ( 4 ) has an upper end ( 11 ) connectable to a drilling apparatus ( 13 ) and fluid source ( 16 ) and a lower end ( 18 ) with an attached casing expander unit ( 20 ) that communicates the fluid passage ( 80 ) of the upper ( 4 ) and lower ( 6 ) drill strings. An upper end ( 22 ) of the lower drill string ( 6 ) is formed from a casing string ( 24 ) telescoped over the casing expander unit. A constriction ( 31 ) connects the casing string ( 24 ) with the rest of the lower drill string ( 6 ). A flow restriction device ( 35 ) in the lower drill string acts to control fluid flow through the fluid passage. In use, the flow restriction device creates a pressure in the fluid passage that acts at the constriction to advance the lower drill string past the upper drill string while simultaneously expanding the portion of the casing string moving past the expander unit.

FIELD OF THE INVENTION

This invention relates to drilling tools and methods and is specificallyconcerned with a casing drilling system in which a casing string is runinto the wellbore with the drilling string and expanded while thedrilling string is in the wellbore.

BACKGROUND OF THE INVENTION

The drilling of wells for oil and gas production conventionally employsrelatively small diameter drilling pipe joined end to end to form adrill string to which is secured the necessary equipment including adrill bit for creating a wellbore which is of larger diameter than thedrilling pipe. After a portion of the wellbore has been drilled, thewellbore is usually lined with a string of tubular casing member joinedend to end to define a casing string. This conventional approachrequires a cycle of drilling the wellbore, pulling the drill string outof the wellbore to the surface and running casing into the wellbore. Theprocess is time consuming and costly.

The technique of casing drilling has been developed to address theproblems of conventional drilling. The casing drilling process involvesrunning a casing string into the wellbore with the drilling string.

Using either of the above techniques, a wellbore may be drilled and thencased to a certain depth, and then the drilling apparatus removed. Ifthe depth of the wellbore is ever later to be extended, it is notpossible to reinsert the drilling apparatus into the cased wellborewithout resorting to a smaller diameter casing string. As differentlower segments of the wellbore are drilled, successively smallerdiameter casing strings are required in order to pass through the casingstrings above.

SUMMARY OF THE INVENTION

To address these and other disadvantages of the prior art, applicant hasdeveloped a casing drilling apparatus and method which involvesalternating between drilling and casing expansion operations under twodifferent drilling fluid pressure regimes in order to insert and expandcasing string into the wellbore while the drill string remains in thewellbore. The present invention therefore allows for formation of a“monobore” well with substantially the same diameter over the totaldepth. This is made possible by expanding a portion of casing after itis placed in the wellbore and after it passes through the segment ofcasing before it.

Accordingly, the present invention provides a drilling assemblycomprising:

an upper drill string and a lower drill string;

a fluid passage extending through the upper drill string and the lowerdrill string for distributing fluid to a bottom hole assembly at thelower end of the lower drill string;

the upper drill string having an upper end connectable to a drillingapparatus and fluid source and having a lower end with an attachedcasing expander unit that communicates the fluid passage of the upperdrill string with the lower drill string;

the lower drill string having an upper end formed from a casing stringtelescoped over the casing expander unit;

a constriction connecting the casing string with the rest of the lowerdrill string; and

a flow restriction device in the lower drill string to control flow offluid through the fluid passage;

wherein the flow restriction device is operable to create a pressure inthe fluid passage that acts at the constriction to advance the lowerdrill string past the upper drill string while simultaneously expandingthe portion of the casing string moving past the expander unit.

In another aspect, the present invention provides a method of drilling awellbore comprising the steps of:

forming a drilling assembly comprising an upper drill string and a lowerdrill string which is telescoped over the upper drill string with afluid passage extending through the upper drill string and the lowerdrill string for distributing fluid to a bottom hole assembly at thelower end of the lower drill string;

operating the drilling assembly according to the following cycle:

drilling a segment of a wellbore with the bottom hole assembly;

stopping drilling and retreating the drill string from the end of thesegment of the wellbore;

advancing the lower drill string past the upper drill string andsimultaneously expanding a portion of a casing string at the upper endof the lower drill string by virtue of movement of the casing stringpast the upper drill string; and

repeating the cycle when the lower drill string reaches the end of thesegment of the wellbore until the desired wellbore depth is achieved.

The present invention offers increased drilling speed by reducing thetime spent expanding the casing and eliminating the need to withdraw thedrill string from the wellbore to insert casing. Reduced drilling costsalso result due to a reduction in drilling time and elimination of stepsand equipment used in the conventional drilling process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are illustrated, merely by way ofexample, in the accompanying drawings in which:

FIG. 1 is a schematic cross sectional view of a preferred embodiment ofthe drilling assembly of the present invention in a wellbore for useprimarily in sliding drilling;

FIG. 2 is a detail view of a section of the drill assembly showing thelower end of the upper drill string including the casing expander unitand the constriction in the lower drill string;

FIG. 3 is a detail view of a section of the drill assembly showing aflow restriction device for controlling fluid flow within the drillstring; and

FIG. 4 is a detail view of a section of the drill assembly showing thebottom hole assembly including the downhole motor and the drill bit in aposition retreated from the bottom of a pilot hole which defines an endof a drilled segment of a wellbore.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, in referring to the position of componentsin the drill strings, “above”, “up”, “upper” and the like describerelative positions closer to the ground surface while “below”, “down”,“lower” and the like describe relative positions closer to the bottom ofthe wellbore.

Referring to FIG. 1, there is shown in schematic form an embodiment of adrilling system according to the present invention intended for slidingdrilling. The drilling assembly 2 comprises an upper drill string 4 anda lower drill string 6 adapted for insertion into a wellbore 8 createdby the drilling assembly. The lower end 9 of lower drill string 6includes a bottom hole assembly (BHA) 10 which includes a drill bit 12,for example, a roller cone bit. A fluid passage 80 extends through upperdrill string 4 and lower drill string 6 for distributing drilling fluid,also, for example, known as drilling “mud”, to bottom hole assembly 10to permit operation of drill bit 12. Upper drill string 4 has an upperend 11 that is connectable to and supported by a drilling apparatus 13such as a derrick at a surface 15. The surface 15 maybe any surface fromwhich drilling may be conducted, including a ground surface or anoffshore drilling platform.

Drilling fluid from fluid source 16 is introduced under pressure intothe fluid passage 80 via port 14. Used fluid exits the lower drillstring 6 at drill bit 12 and serves to lubricate and cool the bit. Theused fluid mixed with material dislodged by drill bit 12 drill flowsupwards as indicated by arrows 17 through wellbore 8 in the annularpassage external to the drill strings 4 and 6. This annular passage issealed at surface level to permit collection of the used fluid forfiltering and recycling through reservoir 16.

As best shown in FIG. 2, the lower end 18 of upper drill string 4opposite supported upper end 11 includes an attached casing expanderunit 20. The casing expander unit includes a passage 20 a therethroughthat communicates the fluid passage 80 of upper drill string 4 with thefluid passage 80 of lower drill string 6.

The upper end 22 of lower drill string 6 is formed from a casing string24 that is telescoped over casing expander unit 20 such that the lowerdrill string 6 essentially “hangs” on the casing expander unit 20. Thelower drill string 6 is maintained in place due to friction plus staticpressure between casing string 24 and casing expander unit 20.Alternatively, the lower drill string 6 may be connectable to andsupported by the drilling apparatus 13 in similar manner as the upperdrill string 4.

The casing expander unit 20 may be comprised of any device, structure orapparatus over which the casing can be moved in order to expand thecasing.

In the preferred embodiment, casing expander unit 20 is formed with agenerally frusto-conical shoulder 26 that expands outwardly downwardlyand forces casing that is moved downwardly past the unit to expandoutwardly. Shoulder 26 is shaped and dimensioned to impart an expandingforce to a casing that is moved over the shoulder. The expanding forcedeforms a casing member to a larger internal diameter. In other words,above casing expander unit 20, there is a casing string portion 24 a ofa first diameter, while below the expander unit, there is an expandedcasing string portion 24 b of an enlarged diameter. Casing expander unit20 also preferably includes an annular shoulder 28 spaced apart fromfrusto-conical shoulder 26 to guide movement of the expanding casingstring and to prevent binding of the casing. Shoulder 26 and/or shoulder28 may also act as an upper seal for expanded casing portion 24 b whichfunctions as a section of the drill string fluid passage 80.

Lower drill string 6 includes a constriction 31 that connects theexpanded portion 24 b of casing string 24 with the rest of the lowerdrill string 6 and communicates the fluid passage 80 through theexpanded casing assembly with the fluid passage 80 of the rest of lowerdrill string 6. The constriction 31 may be comprised of any device,structure or apparatus which is capable of providing a narrowingtransition from the casing string to the rest of the lower drill string6. The functions of the constriction 31 are to convert fluid pressurefrom within the fluid passage 80 to a downward force acting on the lowerdrill string 6 and to provide a transition between the casing and therest of the lower drill string 6.

Referring to FIG. 1, in a preferred embodiment the constriction 31 inthe lower drill string 6 is preferably formed by inserting a latchcoupling 71 between the expanded casing string portion and the rest oflower drill string 6. A packer seal 70 is positioned above the latchcoupling 71 to seal the unit and prevent loss of fluid about theexterior of the latch coupling 71.

FIG. 2 is a detail view of the upper end of the lower drill string 6 andillustrates an alternative constriction 31 in the form of a funnelsubassembly 30. Funnel assembly 30 provides a smooth transition thatconnects the expanded portion 24 b of casing string 24 with the rest ofthe lower drill string 6. As with the latch coupling 71 arrangement, thefluid passage 80 extending through upper drill string 4 is communicatedwith the lower drill string 6 via funnel assembly 30.

A pup joint may be used to connect constriction 31 with the rest of thelower drill string 6.

As shown in FIGS. 1 and 3, lower drill string 6 also includes a flowrestriction device 35 to control flow of fluid through the fluid passage80 and control overall operation of the drilling assembly.

When the drilling apparatus of the present invention is operated toexpand casing, flow restriction device 35 is operated to restrict flowand create an elevated pressure in the fluid passage 80 above the flowrestriction device that acts at constriction 31 and at flow restrictiondevice 35 to advance lower drill string 6 past upper drill string 4while simultaneously expanding the portion of casing string 24 movingpast expander unit 20. In contrast, when drilling, the flow restrictiondevice 35 is set to permit substantially unrestricted flow of drillingfluid to drill bit 12. In other words, flow restriction device 35operates as a bi-pressure subassembly to create two pressure regimeswithin the drill strings 4 and 6 to switch the drilling assembly betweena drilling mode and a casing insertion and expansion mode. The drillingassembly alternates between these two modes to perform its work.

Some development work has been done directed to the notion ofsimultaneously drilling and expanding the casing by always operating ina high flow, high pressure mode. This technique is not consideredworkable since the high pressures required for casing expansion areincompatible with lower pressures which are suitable and safe fordrilling. Also, the rate of casing expansion is expected to be at leastan order of magnitude greater than the drilling penetration mode,depending on conditions, and the forces required for these two modes ofoperation are likewise incompatible. An important feature of the presentinvention is the provision of two different pressure regimes in thefluid passage 80 that allow for alternating between the drilling modeand the casing insertion and expansion modes instead of performing theseoperations simultaneously.

The flow restriction device 35 may be comprised of any structure, deviceor apparatus which is capable of alternately providing two differentpressure regimes in the drill strings 4 and 6. The flow restrictiondevice 35 may be configured to be actuated between the pressure regimesin any manner. For example, the flow restriction device 35 may beactuated by longitudinal or rotational manipulation of the drillingstrings 4 and 6 or by pressure or flow variations of drilling fluid inthe fluid passage 80.

One device suitable for use as a flow restriction device 35 in thepresent invention is a bi-pressure subassembly which includes a barrelcam with detents which is movable between positions to control flow offluid through the unit. The barrel cam is activated by pressure changesin the fluid introduced by cycling the pumps that pump the fluid. Oneexample of equipment that could be adapted to function as a bi-pressuresubassembly is the Adjustable Gauge Stabilizer (AGS™) manufactured bySperry-Sun Drilling Services. The operation of this subassembly isdescribed in the Adjustable Gauge Stabilizer (AGS™) Operations manualwhich is incorporated herein by reference.

U.S. Pat. No. 6,158,533 to Gillis et al. discloses an Adjustable GaugeDownhole Drilling Assembly (Adjustable Gauge Motor (AGM™))that includesa similar barrel cam apparatus and is also incorporated herein byreference.

As adapted for use in the present invention, the AGS™ and the AGM™ areboth able to operate in both an unrestricted fluid flow mode and arestricted fluid flow mode to switch the drilling assembly betweendrilling mode and casing insertion and expansion mode, respectively.

Depending upon the application of the invention and the design of thebottom hole assembly 10, a flow restriction device 35 which comprises anapparatus similar to that of the AGS™ or the AGM™ may or may not includethe function of an adjustable gauge stabilizer. In other words, thestructures of the AGS™ and the AGM™ are adapted for use with theinvention primarily because of their capability to provide twoalternating pressure regimes in the drill strings 4 and 6.

FIG. 3 provides a detail section view through an AGS™ subassembly whichincludes a barrel cam actuator and a movable orifice to control fluidflow through the subassembly. Additional detail of these and othercomponents of this embodiment of flow restriction device 35 may beobtained from the documents which are incorporated by reference.

A second device which is potentially suitable to be adapted for use asthe flow restriction device 35 is disclosed in U.S. Pat. No. 6,439,321to Gillis et al for a Piston Actuator Assembly for an Orienting Device.This device comprises a longitudinally movable piston which provides afirst partial obstruction and a flow restrictor which provides a secondpartial obstruction. The first partial obstruction and the secondpartial obstruction may be selectively aligned or misaligned to providetwo different pressure regimes. U.S. Pat. No. 6,439,321 is incorporatedherein by reference.

Referring to FIG. 4, there is shown a preferred arrangement of a bottomhole assembly 10 for use with the drilling assembly of the presentinvention. The bottom hole assembly includes a downhole drilling motor50 that is operated by fluid pressure, an underreamer 52, a stabilizer54, a near-bit stabilizer 56 and drilling bit 12. This particular bottomhole assembly is intended for sliding drilling due to the presence ofdownhole motor 50.

It will be appreciated by those skilled in the art that not all thecomponents of bottom hole assembly 10 illustrated in FIG. 4 arenecessarily required in all applications of the drilling system of thepresent invention. For example, it may not always be necessary to havean underreamer or stabilizers. In addition, the stabilizers may bedifferent in number and in position within the bottom hole assembly. Thebottom hole assembly can also include subassemblies for steering thedrill bit in directional drilling applications. The illustrated anddescribed embodiments of the present invention are directed toessentially vertical wellbores. It will be apparent to one skilled inthe art that the drilling system of the present invention can be used innon-vertical drilling applications.

In addition, measurement-while-drilling (MWD) systems can be used withthe drilling apparatus of the present invention. Typically, such systemsare used to sense and communicate properties such as drillingtemperatures, pressures, azimuth and inclination and would be installedin the lower drill string 6 above bottom hole assembly 10 to readilytransmit data from the wellbore 8 to the surface.

When used in conjunction with the sliding drilling bottom hole assembly10 described above and illustrated in FIG. 4, the bi-pressuresubassembly 35 is cycled “Pumps Off-Pumps On” to shift the unit intofull flow, low backpressure operation with substantially unrestrictedflow of drilling fluid through the subassembly. The subassembly isselected to be of sufficient size and rating to handle the flow volumeand pressure. The flow of drilling fluid through the bi-pressuresubassembly drives motor 50, deploys the cutter arms on underreamer 52and supplies coolant fluid to drill bit 12 in order to drill ahead intopilot hole 40 by advancing upper drill string 4 and lower drill string 6together into the wellbore 8. New drill joints are added to the upperend 11 of upper drilling string 4 and new casing joints are added to theupper end 22 of lower drilling string 6 as the drilling assembly is fedinto the wellbore 8. The downward force on drill bit 12 or weight on bit(WOB) is provided primarily by the weight of the drilling strings abovethe drill bit. At low pressure, relative movement between the upperdrill string 4 and the lower drill string 6 is prevented by the frictionbetween casing expander unit 20 and casing string 24 and by the fluidpressure exerted on constriction 31 by passage of the drilling fluidthrough the drilling strings 4 and 6.

Once a segment of the wellbore 8 has been drilled a desired distance,the bi-pressure subassembly 35 is cycled by a “Pumps Off-Pumps On”sequence of the pumps at the surface supplying the drilling fluid toshift the unit into high backpressure operation in which fluid flow isreduced to the motor, underreamer and bit to such an extent that thesecomponents stop functioning. It is contemplated that the flow throughthe bi-pressure subassembly 35 in this restricted flow position will beextremely small. In other words, the passage through the subassemblywill be very small in the restricted flow position. This can be achievedby selecting an appropriate orifice size for the subassembly.

With drilling halted by stopping of the drill bit, the drill strings 4and 6 are retracted from the surface to retreat drill bit 12 from thebottom 42 of the pilot hole 40. This position of drill strings 4 and 6is shown in FIGS. 1 and 4. Preferably the drill strings 4 and 6 areretracted before a high pressure regime is created in the drill strings4 and 6 so that the lower drill string 6 is not inadvertently impactedagainst the bottom of the wellbore 8. In other words, preferably thedrill strings 4 and 6 are retreated after the “Pumps Off” portion butbefore the “Pumps On” portion of the “Pumps Off-Pumps On” sequence.

When using the bottom hole assembly 10 illustrated in FIG. 4, the drillstrings 4 and 6 are preferably only lifted far enough so that near bitstabilizer 56 remains at least partially located in pilot hole 40 toensure that the lower drill string 6 remains centred in the fullwellbore 8. Backpressure builds in the fluid passage 80 above thebi-pressure subassembly 35, and is allowed to reach a level sufficientto begin pushing lower drill string 6 back towards the bottom 42 ofpilot hole 40. As best shown in FIG. 2, increased pressure is exertedequally in all directions at constriction 31, however, since the upperdrill string 4 is held stationary with respect to the surface, pressureforces at the lower end of the funnel result in a net downward forcebeing exerted at constriction 31 and at flow restriction device 35 asindicated by arrow 33 (arrows not shown for 35). Referring to FIG. 1,casing expander unit 20 is held stationary with respect to the surface15 by virtue of being attached to the lower end 18 of upper drill string4 which is supported by drill derrick 13. To accommodate downwardmovement of the lower drill string 6, a length of the upper casingportion 22 of lower drilling string 6 must telescope past conicalshoulder 26 of casing expander unit 20 which causes expansion of thecasing to an enlarged internal diameter.

While the illustration of FIG. 4 shows a relatively short length ofpilot hole 40, it is contemplated that the drilling phase can beconducted over distances on the order of hundreds of feet or more beforedrilling is stopped, the drill bit is retreated and casing is insertedand expanded over the length of the newly created section.

In most applications, it is preferable that sealed junctions be providedbetween adjacent segments of casing string 24 In other words, the upperend of a lower segment of casing should preferably be sealinglyconnected to the lower end of an upper segment of casing. This can beaccomplished as a lower segment of casing is expanded, and may involvethe use of a rubber cladding on the surfaces of the casing at the endsof the casing. These techniques are already extant in the prior art.

This sealed junction is optional and may not always be required. Infact, in some applications, there may actually be gaps in the boreholebetween segments of casing.

Fluid flow through the fluid passage 80 is stopped to halt the downwardmovement of lower drill string 6 and expansion of the casing beforedrill bit 12 reaches the bottom 42 of pilot hole 40. This may, forexample, be achieved by initiating a further “Pumps Off-Pumps On”sequence in order to initiate the drilling of a further segment ofwellbore 8. Alternatively, fluid flow may simply be stopped tofacilitate an interruption in drilling and casing expansion operations.

There is a potential danger of accidentally “tagging bottom” with thedrill bit and underreamer assemblies traveling at full casing insertionand expansion speed. To prevent damage to these components, which wouldsignificantly disrupt the entire drilling operation, it is preferable toprovide safeguards against this potential danger. Such safeguards mayinclude a device, structure or apparatus for dissipating pressure withinthe fluid passage 80 in response to an occurrence of tagging bottom or adevice, structure or apparatus for absorbing the impact associated withan occurrence of tagging bottom. One or both of these safeguards may beprovided and may be provided in one or a plurality of devices,structures or apparatus.

In a preferred embodiment, both safeguards are provided in a singleapparatus, which apparatus comprises a shock absorbing unit 60 locatedabove bi-pressure subassembly 35, preferably in the lower drill string6. Such a unit is shown schematically in FIG. 1. Preferably, shockabsorbing unit 60 is a modified shock tool which acts to relievepressure on contact. Unit 60 includes a spring biased piston whichnormally covers relief ports. As the shocktool compresses when the drillbit is moved against the bottom of the hole, the springs compress, thepiston moves, and the ports become exposed, thus releasing fluid fromthe fluid passage 80 as shown by arrows 72. The escape of fluidinstantly reduces the backpressure and hence the downward pressureexerted at constriction 31, thereby interrupting the casing insertionand expansion process.

Details of a conventional two-way shock tool or shock absorbing toolwhich could be adapted for use with the invention can be found inCanadian Patent No. 1,226,274 to Wenzel, which is incorporated herein byreference.

Other mechanisms could be used to accomplish the goal of providingsafeguards against damage to the bottom hole assembly 10 due to impactunder high fluid pressure. Safeguards directed at dissipating thepressure within the fluid passage 80 should generally be located abovethe flow restriction device 35 (in either the upper drill string 4 orthe lower drill string 6).

Safeguards directed at absorbing the impact of the drill bit 12 at theend of the wellbore 8 may be located at any position in the upper drillstring 4 or the lower drill string 6 but are preferably located in thelower drill string 6 in relative close proximity to the bottom holeassembly 10. Where both safeguards are integrated in a single device,structure or apparatus, this device, structure or apparatus shouldtherefore be located above the flow restriction device 35 in either theupper drill string 4 or the lower drill string 6.

A particular advantage of the drilling apparatus and method of thepresent invention is that it permits the resumption and extension of awellbore 8 that has already been cased to a certain depth withoutintroducing progressively reduced diameter sections. Using conventionaldrilling techniques, it is not possible to reinsert the drillingapparatus into the cased wellbore 8 without resorting to a smallerdiameter casing string. As different lower segments of the wellbore 8are drilled, successively smaller diameter casing strings are requiredin order to pass through the casing strings above. With the apparatusand method of the present invention, it is possible to installsubsequent casing strings in each new section as the casing strings aremovable through the existing pre-expanded wellbore 8 for expansion afterthey are positioned in the newly drilled portion of the wellbore 8. Whenthe drilling assembly of the present invention is used in this manner toextend an existing cased well, upper drill string 4 is extended into thewell to a point adjacent the end of the installed casing to position thecasing expansion unit 20 to begin expansion of the new casing string ata location that preferably results in some overlap of the casingstrings.

In practice, it is sometimes necessary to retrieve the bottom holeassembly 10 from the end of the drilling strings if a component breaksor if drilling is completed. If constriction 31 is formed from latchcoupling 71, the latch coupling 71 provides a convenient point ofretrieval for the bottom hole assembly to facilitate removal. Onealternative retrieval mechanism that can be incorporated in the bottomhole assembly of the present invention is described in U.S. Pat. No.5,197,553 (Leturno) or U.S. Pat. No. 5,271,472 (Leturno) which areincorporated herein by reference. A second alternative retrievalmechanism is also discussed in U.S. Pat. No. 5,472,057 (Winfree) whichis also incorporated herein by reference. Other retrieval mechanisms forthe bottom hole assembly or portions thereof may also be used with theinvention.

The foregoing description primarily details a drilling system accordingto the present invention that relies on a sliding drilling arrangementusing a downhole drilling motor 50 as shown in FIG. 1. It will beappreciated that the present invention is not limited to thisarrangement. The drilling system can also be used in a rotary drillingarrangement in which the lower drill string 4 or both drill strings 4and 6 are rotated from the surface.

In the rotary drilling arrangement, downhole motor 50 may not berequired. Instead, the drill bit 12 may be driven by rotation of eitheror both of the drill strings 4 and 6.

If both the upper drill string 4 and the lower drill string 6 are to berotated, then consideration must be given to ensuring that the drillstrings 4 and 6 rotate together. The frictional forces between the upperdrill string 4 and the lower drill string 6 at the location of thecasing expander unit 20 may or may not be sufficient to transmit torquefrom the upper drill string 4 to the lower drill string 6. It maytherefore be necessary either to rotate both of the drill strings 4 and6 simultaneously from the surface or to provide a more positivemechanism for ensuring that torque can be transmitted from the upperdrill string 4 to the lower drill string 6. Such a mechanism maycomprise a latch mechanism or splines, ridges or grooves in engagingsurfaces of the upper drill string 4 and the lower drill string 6.

Alternatively, if only the lower drill string 6 is to be rotated duringrotary drilling, a bearing assembly (not shown) at casing expander unit20 would be required to accommodate rotation of the casing stringrelative to the casing expander unit 20 when in drilling mode.

The invention may also be utilized with a combination of rotary drillingand sliding drilling techniques by combining the features of both thesliding drilling embodiments and the rotary drilling embodiments asdescribed above and by incorporating a downhole motor 50 in the bottomhole assembly 10 even where rotary drilling is contemplated.

While a downhole motor 50 in the bottom hole assembly 10 may beunnecessary in a rotary drilling arrangement, a drilling fluidrestriction device 35 is still required to provide lubricating drillingfluid to the drill bit during drilling mode and to develop the necessaryhigh pressure in the fluid passage 80 to permit expansion of the casingduring casing expansion mode.

It may, however, be possible for some applications of the invention toeliminate the constriction 31 if sufficient force can be developed atthe flow restriction device 35 to permit expansion of the casing duringcasing expansion mode. This possibility depends upon the extent to whichthe flow restriction device 35 restricts flow in the fluid passage 80when the flow restriction device 35 is in casing expansion mode. Thispossibility also depends upon the ability to provide a transitionbetween the casing and the rest of the lower drill string 6 without theconstriction 31.

Alternatively, it may be possible to combine the functions of theconstriction 31 and the flow restriction device 35 at a single locationin the lower drill string 6 instead of at longitudinally spacedlocations. An integrated constriction 31 and flow restriction device 35could for example provide a transition between the casing and the restof the lower drill string 6, convert fluid pressure within the fluidpassage 80 to a downward force acting on the lower drill string 6, andprovide for two different pressure regimes.

Although the present invention has been described in some detail by wayof example for purposes of clarity and understanding, it will beapparent that certain changes and modifications may be practised withinthe scope of the appended claims.

1. A drilling assembly comprising: an upper drill string and a lowerdrill string; a fluid passage extending through the upper drill stringand the lower drill string for distributing fluid to a bottom holeassembly at the lower end of the lower drill string; the upper drillstring having an upper end connectable to a drilling apparatus and fluidsource and having a lower end with an attached casing expander unit thatcommunicates the fluid passage of the upper drill string with the lowerdrill string; the lower drill string having an upper end formed from acasing string telescoped over the casing expander unit; a constrictionconnecting the casing string with the rest of the lower drill string;and a flow restriction device in the lower drill string to control flowof fluid through the fluid passage; wherein the flow restriction deviceis operable to create a pressure in the fluid passage that acts at theconstriction to advance the lower drill string past the upper drillstring while simultaneously expanding the portion of the casing stringmoving past the expander unit.
 2. A drilling assembly as claimed inclaim 1 in which the drilling assembly is a sliding drilling assemblyand the bottom hole assembly includes a downhole motor driven by fluidto rotate an attached drill bit.
 3. A drilling assembly as claimed inclaim 2 in which the bottom hole assembly includes a reamer subassembly.4. A drilling assembly as claimed in claim 2 in which the bottom holeassembly includes at least one stabilizer subassembly.
 5. A drillingassembly as claimed in claim 2 in which the downhole motor is positionedafter the flow restriction device which is used to control fluid flow tothe motor.
 6. A drilling assembly as claimed in claim 1 in which thedrilling assembly is a rotary drilling assembly and the bottom holeassembly includes a drill bit that is rotated by rotation of the upperand lower drill strings.
 7. A drilling assembly as claimed in claim 1 inwhich the bottom hole assembly includes a drill bit that is drivable bya combination of rotary drilling from the surface and a downhole motorin the bottom hole assembly.
 8. A drilling assembly as claimed in claim1 including a pressure relief unit in the lower drill string before theflow restriction device.
 9. A drilling assembly as claimed in claim 1including a shock absorbing unit in the lower drill string before theflow restriction device.
 10. A drilling assembly as claimed in claim 1in which the constriction is formed at a latch coupling.
 11. A drillingassembly as claimed in claim 10 in which the latch coupling is sealed bya packer seal.
 12. A method of drilling a wellbore comprising: forming adrilling assembly comprising an upper drill string and a lower drillstring which is telescoped over the upper drill string with a fluidpassage extending through the upper drill string and the lower drillstring for distributing fluid to a bottom hole assembly at the lower endof the lower drill string; operating the drilling assembly according tothe following cycle; drilling a segment of a wellbore with the bottomhole assembly; stopping drilling and retreating the bottom hole assemblyfrom an end of the segment of the wellbore; advancing the tower drillstring past the upper drill string and simultaneously expanding aportion of a casing string at the upper end of the lower drill string byvirtue of movement of the casing string past the upper drill string;repeating the cycle when the lower drill string reaches the end of thesegment of the wellbore until the desired wellbore depth is achieved.13. A method as claimed in claim 12 in which advancing the lower drillstring past the upper drill string includes controlling the flow offluid in the fluid passage to increase the pressure in the drill stringto cause relative movement of the lower drill string with respect to theupper drill string and to expand the casing string.
 14. A method asclaimed in claim 13 wherein controlling fluid flow in the fluid passageinvolves actuating a flow restriction device in the lower drill stringto create increased pressure in the drill string above the flowrestriction device.
 15. A method as claimed in claim 13 includingforming a constriction in the lower drill string to provide a locationfor the pressure to exert a net downward force on the lower drillString.
 16. A method as claimed in claim 13 including providing a casingexpander unit at a lower end of the upper drill string which acts toexpand the portion of the casing string moving past the expander unit.17. A drilling assembly comprising: an upper drill string and a lowerdrill string; a fluid passage extending through the upper drill stringand the lower drill string for distributing fluid to a bottom holeassembly at a lower end of the lower drill string; the upper drillstring having an upper end connectable to a drilling apparatus and fluidsource and having a lower end with an attached casing expander unit thatcommunicates the fluid passage of the upper drill string with the lowerdrill string; the lower drill string having an upper end formed from acasing string telescoped over the casing expander unit, wherein thecasing string is connected with a remainder of the lower drill stringforming the lower end of the lower drill string; a constrictionconnecting the casing string with the remainder of the lower drillstring; and a flow restriction device in the lower drill string tocontrol a flow of fluid through the fluid passage; wherein the flowrestriction device is alternately actuatable between a first Pressureregime to provide a drilling mode of the drilling assembly wherein theflow of fluid through the fluid passage to the bottom hole assembly issubstantially unrestricted and a second pressure regime to provide acasing insertion and expansion mode of the drilling assembly wherein theflow of fluid through the fluid passage to the bottom hole assembly isrestricted, thereby creating a pressure in the fluid passage that actsat the constriction to advance the lower drill string past the upperdrill string while simultaneously expanding the portion of the casingstring moving past the expander unit, and wherein actuation of the flowrestriction device switches the drilling assembly between the drillingmode and the casing insertion and expansion mode.
 18. A drillingassembly as claimed in claim 17 in which the drilling assembly is asliding drilling assembly and the bottom hole assembly includes adownhole motor driven by fluid to rotate an attached drill bit.
 19. Adrilling assembly as claimed in claim 18 in which the downhole motor ispositioned after the flow restriction device which is used to controlfluid flow to the motor.
 20. A drilling assembly as claimed in claim 17in which the drilling assembly is a rotary drilling assembly and thebottom hole assembly includes a drill bit that is rotated by rotation ofthe upper and lower drill strings.
 21. A drilling assembly as claimed inclaim 17 in which the bottom hole assembly includes a drill bit that isdrivable by a combination of rotary drilling from the surface and adownhole motor in the bottom hole assembly.
 22. A drilling assembly asclaimed in claim 17 wherein the bottom hole assembly is comprised of adrill bit for drilling a wellbore.
 23. A drilling assembly as claimed inclaim 22 wherein the remainder of the lower drill string is comprised ofthe flow restriction device.
 24. A drilling assembly as claimed in claim23 wherein the flow restriction device is comprised of a bi-pressuresubassembly alternately actuatable between the first pressure regime andthe second pressure regime.
 25. A drilling assembly as claimed in claim24 wherein the flow of fluid through the fluid passage to the bottomhole assembly is substantially obstructed upon actuation of thebi-pressure subassembly to the second pressure regime to provide thecasing insertion and expansion mode.
 26. A drilling assembly as claimedin claim 24, wherein the lower drill string is further comprised of apressure relief unit for dissipating pressure within the fluid passage.27. A drilling assembly as claimed in claim 26 wherein the pressurerelief unit is positioned between the casing string and the flowrestriction device.
 28. A drilling assembly as claimed in claim 24,wherein the lower drill string is further comprised of a shock absorbingunit for absorbing any impact of the bottom hole assembly.
 29. Adrilling assembly as claimed in claim 28 wherein the remainder of thelower drill string is comprised of the shock absorbing unit.
 30. Adrilling assembly as claimed in claim 28 wherein the shock absorbingunit is positioned between the casing string and the flow restrictiondevice.
 31. A drilling assembly as claimed in claim 24 wherein theconstriction is formed at a latch coupling connected between the casingstring and the remainder of the lower drill string, and wherein thefluid passage extends through the latch coupling.
 32. A drillingassembly as claimed in claim 24 wherein the constriction is comprised ofa funnel subassembly connected between the casing string and theremainder of the lower drill string, wherein the fluid passage extendsthrough the funnel subassembly.
 33. A method of drilling a wellborecomprising: forming a drilling assembly comprising an upper drill stringand a lower drill string which is telescoped over the upper drill stringwith a fluid passage extending through the upper drill string and thelower drill string for distributing fluid to a bottom hole assembly atthe lower end of the lower drill string; operating the drilling assemblyaccording to the following cycle: actuating the drilling assembly to adrilling mode and drilling a segment of a wellbore with the bottom holeassembly; stopping drilling and retreating the bottom hole assembly froman end of the segment of the wellbore; actuating the drilling assemblyto a casing insertion and expansion mode and advancing the lower drillstring past the upper drill string and simultaneously expanding aportion of a casing string at the upper end of the lower drill string byvirtue of movement of the casing string past the upper drill string;repeating the cycle when the lower drill string reaches the end of thesegment of the wellbore until the desired wellbore depth is achieved.34. A method as claimed in claim 33 in which actuating the drillingassembly to the casing insertion and expansion mode and advancing thelower drill string past the upper drill string includes controlling theflow of fluid in the fluid passage to increase the pressure in the fluidpassage and to thereby cause relative movement of the lower drill stringwith respect to the upper drill string and to expand the casing string.35. A method as claimed in claim 34 wherein controlling fluid flow inthe fluid passage includes actuating a flow restriction device in thelower drill string to create increased pressure in the fluid passageabove the flow restriction device.
 36. A method as claimed in claim 34including forming a constriction in the lower drill string to provide alocation for the pressure to exert a net downward force on the lowerdrill string.
 37. A method as claimed in claim 34 including providing acasing expander unit at a lower end of the upper drill string which actsto expand the portion of the casing string moving past the expanderunit.
 38. A method as claimed in claim 34 wherein the lower drill stringincludes a flow restriction device for controlling a flow of fluidthrough the fluid passage and wherein the step of actuating the drillingassembly to the drilling mode is comprised of actuating the flowrestriction device to a first pressure regime wherein the flow of fluidthrough the fluid passage to the bottom hole assembly is substantiallyunrestricted.
 39. A method as claimed in claim 38 wherein actuating thedrilling assembly to the casing insertion and expansion mode iscomprised of actuating the flow restriction device to a second pressureregime wherein the flow of fluid through the fluid passage to the bottomhole assembly is restricted such that a pressure is created in the fluidpassage to thereby cause relative movement of the lower drill stringwith respect to the upper drill string and to expand the casing string.40. A method as claimed in claim 39 further comprising forming aconstriction in the lower drill string to provide a location for thepressure in the fluid passage to exert a net downward force on the lowerdrill string.
 41. A method as claimed in claim 40 further comprisingproviding a casing expander unit at a lower end of the upper drillstring which acts to expand the portion of the casing string moving pastthe expander unit.
 42. A method as claimed in claim 34 wherein actuatingthe drilling assembly to the casing insertion and expansion mode iscomprised of actuating the flow restriction device to a second pressureregime wherein the flow of fluid through the fluid passage to the bottomhole assembly is restricted such that a pressure is created in the fluidpassage to thereby cause relative movement of the lower drill stringwith respect to the upper drill string and to expand the casing string.43. A method as claimed in claim 42 further comprising forming aconstriction in the lower drill string to provide a location for thepressure in the fluid passage to exert a net downward force on the lowerdrill string.
 44. A method us claimed in claim 43 further comprisingproviding a casing expander unit at a lower end of the upper drillstring which acts to expand the portion of the casing string moving pastthe expander unit.